1. Field of the Invention
This invention relates to novel methods and apparatus for applying heat to articles and materials during processing thereof.
2. Description of the Prior Art
Many manufacturing processes require that certain articles or materials produced be heated in a very precise manner. For example, in the manufacture of welded seam cans to be used for containing food, it is necessary to apply a coating of lining material along the weld seam inside the cans and then to heat the material to cure it. In the interest of speed it is desired to heat the lining material rapidly; however, if the material is heated too rapidly it is likely to blister or deteriorate. It is also important that the heating be confined to the lining material in the region along the weld seams and that it not extend over the entire can. In other manufacturing processes precise amounts of heat at predetermined temperatures must be applied to moving sheets or webs. For example, in the processing of paper or other webs, it is often necessary to apply heat for curing, drying or otherwise treating the web. Where the webs are thin the amount of heat applied to them must be closely controlled and the temperature at which the heat is applied must be maintained within close tolerances, otherwise the web may be ruined.
Direct flame heaters, such as torches, are well known. Direct flame heaters, however, are often not suitable for controlled heating of various articles and materials because of the wide variation of temperature that exists over a small region in the vicinity of the flame. For example, the temperature at the flame tip of a gas burner may be in excess of 1200.degree. C., but just a few millimeters away from the flame tip the temperature may be only a few hundred degrees centigrade.
To overcome the problem of sharp temperature variations in the vicinity of a flame, indirect flame heating arrangements have been employed for most industrial processes. U.S. Pat. Nos. 3,437,322 and 3,541,190 show improvements in indirect flame heating applications. In an indirect flame heating arrangement a flame heater is used to heat a current of air flowing along a duct and the heated air is directed through the duct to the location where heat is to be applied.
Although indirect flame heating arrangements provide uniform temperature over a much larger area than is possible with a gas flame heater, the ducting required for such indirect heating tends to be bulky, especially where it must be insulated. Also, because of the time required for the air to flow through the ducting between the flame heater and the region to be heated, it is not possible to make instantaneous adjustments of temperature or heat output from a conventional indirect flame heater.
Other types of flame heaters which utilize so called "secondary air" to mix with and assist in flame combustion are also well known; and examples of these are seen in U.S. Pat. Nos. 3,794,459, 4,048,927, 4,124,353, 4,156,394, 4,175,919, 4,235,591, 4,278,418 and 4,297,093. None of the devices of those patents however are capable of applying controlled amounts of heat at an even temperature to a precise area.